The electrode potential of M^{2+}/M of 3d-series elements shows positive value for
(a) Fe
(b) Co
(c)

Q: Why does Copper have a positive $E^\circ$ value while the rest of the 3d series is negative?

The standard electrode potential depends on three factors: enthalpy of atomization, ionization enthalpy, and hydration enthalpy. For Copper, the high energy required to atomize and ionize it is not fully compensated by the energy released during hydration. Hence, the overall process is less favorable, resulting in a positive $E^\circ$.

Q: Does this mean Copper cannot be oxidized at all?

Not at all! Copper can be oxidized, but it requires a stronger oxidizing agent than the $H^+$ ion. For example, it reacts with oxidizing acids like concentrated Nitric Acid ($HNO_3$) or hot concentrated Sulfuric Acid ($H_2SO_4$), where the nitrate or sulfate ions act as the oxidizing agents instead of hydrogen.

Q: Are there other transition metals with positive $E^\circ$ values?

Yes, but not in the 3d series. In the 4d and 5d series, metals like Silver (Ag), Gold (Au), and Platinum (Pt) also have positive standard reduction potentials. They are known as 'noble metals' because of their low reactivity.

Q: Do I need to memorize the exact $E^\circ$ values for the JEE exam?

You don't need to memorize the exact numerical values, but you absolutely must know the trends and the exceptions. Knowing that Copper is the only 3d element with a positive value is a highly tested fact.

Q: What happens if I connect a Copper half-cell to a Standard Hydrogen Electrode (SHE)?

Because Copper has a higher reduction potential ($+0.34$ V) than Hydrogen ($0.00$ V), Copper will act as the cathode (where reduction occurs), and the SHE will act as the anode (where oxidation occurs). Electrons will flow from the Hydrogen electrode to the Copper electrode.

Question

The electrode potential of $M^{2+}/M$ of 3d-series elements shows positive value for

Options:

(A) Fe

(B) Co

(C) Zn

(D) Cu

Sigma Percentile · Accepted Answer

### The Electrochemical Arena

When we dive into the world of electrochemistry, the Standard Reduction Potential ($E^\circ$) acts as our ultimate guide. It tells us a simple story: how badly does a metal ion want to grab electrons and turn back into a solid metal?

By convention, we measure everything against the Standard Hydrogen Electrode (SHE), which is arbitrarily assigned a potential of exactly $0.00 	ext{ V}$. If a metal has a **negative** $E^\circ$ value, it means it is a stronger reducing agent than hydrogen. It prefers to lose electrons (oxidize) and will happily dissolve in dilute acids to liberate hydrogen gas.

### The Rule of the 3d Series

Let's look at the first row of transition metals, the 3d series. From Scandium to Zinc, almost all of these metals are quite reactive. They have negative standard reduction potentials.

For instance, Zinc has an $E^\circ_{Zn^{2+}/Zn}$ of $-0.76 	ext{ V}$, Iron is at $-0.41 	ext{ V}$, and Cobalt sits at $-0.28 	ext{ V}$. Because their potentials are negative, they have a higher tendency to form ions in solution than hydrogen does. If you drop a piece of zinc into hydrochloric acid, it will immediately start bubbling, releasing $H_2$ gas.

### The Copper Exception

But then, we hit a roadblock: **Copper (Cu)**. Copper is the rebel of the 3d series. Its standard reduction potential is **positive** ($E^\circ_{Cu^{2+}/Cu} = +0.34 	ext{ V}$).

What does this positive value mean physically? It means that copper ions ($Cu^{2+}$) have a higher tendency to get reduced than hydrogen ions ($H^+$). Consequently, solid copper has a very low tendency to oxidize. If you drop a copper coin into dilute $HCl$, absolutely nothing happens. Copper refuses to displace hydrogen.

### The Thermodynamic Secret

Why is copper so unique? To understand this, we have to break down the process of a solid metal turning into an aqueous ion into three thermodynamic steps:

1. **Atomization:** The solid metal must be broken apart into gaseous atoms (requires energy, $\Delta H_{atom}$). Copper has a very high enthalpy of atomization.
2. **Ionization:** The gaseous atoms must be stripped of two electrons to form $M^{2+}$ ions (requires energy, $IE_1 + IE_2$). Copper's ionization enthalpies are quite high.
3. **Hydration:** The gaseous ions are dropped into water, where water molecules surround them, releasing energy (releases energy, $\Delta H_{hyd}$).

For a metal to have a negative $E^\circ$, the energy released during hydration must be greater than the energy consumed during atomization and ionization.

For copper, the sum of its high atomization enthalpy and high ionization enthalpies is so massive that the negative hydration enthalpy simply **cannot compensate** for it. The overall process is endothermic, making the formation of $Cu^{2+}$ energetically unfavorable compared to hydrogen. Thus, copper proudly holds its positive $E^\circ$ value, standing alone in the 3d series!

Animated Solution for Chemistry - d and f-Block Elements: The electrode potential of $M^{2 +} / M$ of 3d-series elements shows positive value for

Select Answer:

Visualized Solution

$E_{M^{2 +} / M}^{\circ} Values$

$Meaning of E^{\circ}$

$Values for Zn, Fe, Co$

$Value for Cu$

$Conclusion$

$Why is Cu positive?$

The Sigma Insight: d-block Elements

The Electrochemical Arena

The Rule of the 3d Series

The Copper Exception

The Thermodynamic Secret

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Animated Solution for Chemistry - d and f-Block Elements: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } The electrode potential of M2+/M of 3d-series elements shows positive value for